EARTH IN SPACE
Key Concepts
How does Earth move in space?
What causes the cycle of seasons on
Earth?
Key Terms
Astronomy Orbit
Axis Calendar
Rotation Solstice
Revolution Equinox
How Earth Moves
• Astronomy –the study of the moon, stars and other
objects in space
• Earth moves through space in two major ways:
rotation and revolution
• Rotation- the spinning of Earth on its axis
• Axis – imaginary line through Earth’s center and North and
South poles
• Rotation causes day and night
• Takes 24 hours for one rotation
• Revolution- movement of one object around
another
• One complete revolution of Earth around the sun is
a year
• Earth follows a slightly elliptical orbit around sun
Rotation vs Revolution
Checkup
How many rotations of Earth take
place in one revolution around the
sun?
Calendars
• A Calendar is system of organizing the
time that defines the beginning, length
and divisions of a year
• People of many cultures have added to our idea
of a calendar
• Egyptians determined that there are 365 days in
a year
• Many people followed moon cycles – there are
about 29 ½ days in a moon cycle or month
• Problem = 12 moon cycles of 30 days only adds
up to 354 days.
Calendar
• Romans borrowed Egyptian calendar of
365 days
• Problem: It actually takes the Earth 365 ¼ days to
complete one revolution. The Romans added 1 day
every 4 years. The 4th year is known as a leap year
• Problem: Roman calendar still off by 11 min. Over
the centuries the minutes added up and by 1500’s
the beginning of spring was coming 10 days early
• Pope Gregory XIII dropped 10 days from the
Calendar to correct. He also made other minor
changes.
• This is the calendar we use today
Check up
What is a leap year?
Explain why it was difficult for ancient
peoples to develop workable calendars
The Seasons on
Earth
Temperate places on Earth have 4 seasons:
Winter, spring, summer, fall
How sunlight hits the Earth affects how warm
an area is.
Sunlight hits more directly and a smaller area
at the equator so it is warmer
Sunlight hits at an angle and over a greater
area at the poles so it is colder
Earth’s Tilted Axis
• Earth has seasons because its axis
is tilted as it revolves around the
sun. (23.5° from vertical)
• As Earth revolved around the sun, the north
end is tilted towards the sun for half of the
year and tilted away the other half
• Summer and winter are caused by the Earth’s
tilt not by changes in Earth’s distance from
the sun
• Earth is actually farthest from the sun during
the northern hemisphere’s summer
Earth in June
North end of Earth’s axis
is tilted toward the sun
Sun is higher in the sky and there are more hours of
sunlight
Sun’s rays are more direct
It is summer in Northern Hemisphere
South end of Earth’s axis is tilted away
Sun is low in the sky, days are shorter than nights
Sun’s rays are more slanted
It is winter in southern hemisphere
Earth in December
Southern Hemisphere receives most direct
sunlight – summer
Northern Hemisphere receives slanted rays -
winter
Solstices
Solstice – when sun is farthest north or south of
the equator
Day when the sun is farthest north of the
equator = about June 21 = Summer Solstice in
the Northern Hemisphere and Winter Solstice in
Southern Hemisphere
Day when the sun is farthest south of the
equator = about December 21= Winter Solstice
in Northern Hemisphere and Summer Solstice in
Southern Hemisphere
Equinoxes
Halfway between the solstices, neither
hemisphere is tilted toward or away from sun
Noon is directly overhead at the equator
Day and night are equal lengths - about 12
hours each
Occurs twice a year around March 21(Vernal
or spring equinox) and September 21
(autumnal equinox)
Solstices and
Equinoxes
Check up
If Earth’s axis were not tilted,
what would happen to the length
of days over the course of a year?